Non-ferrous, intermetallic alloys based on tri-nickel aluminide (Ni.sub.3 Al alloys) possess many properties making them useful for applications involving elevated temperatures. A primary reason for this is the characteristic of these alloys that, contrary to the behavior of conventional alloys, the yield strength of the alloys increases with increasing temperatures. Thus, where yield strength is important in high temperature applications, these alloys are often the preferred material.
To take advantage of this important characteristic of nickel aluminide alloys, many attempts have been made to adapt base alloys to special uses. A specialized alloy designed to improve fabricable materials was proposed in U.S. Pat. No. 4,711,761, assigned to Martin Marietta Energy Systems, Inc. The proposed alloy contained iron for increased yield strength and contained titanium, manganese, and niobium for improved cold fabricability. Iron-containing nickel aluminide alloys have been proposed also containing hafnium and zirconium for improved strength.
Iron containing and some non-ferrous base alloys exhibited brittleness at higher temperatures, however, especially in an oxygen bearing environment. Other problems with existing alloys were pointed out in U.S. Pat. No. 5,006,308. This patent proposed a non-ferrous alloy in which the base alloy was compounded with chromium, zirconium, and boron.
Attempts have also been made to produce castable nickel aluminide alloys for applications in such apparatus as turbocharger rotors. For applications such as this, the yield strength at room temperatures (about 25.degree. C.) is required to be above about 80 ksi (ksi=1,000-lb per square inch.congruent.6.97 MPa). Known alloys were only marginally acceptable. An alloy having improved yield strength at this temperature was proposed in U.S. Pat. No. 5,108,700, in which defined amounts of chromium, zirconium, boron, and either or both of molybdenum and niobium were present.
Castable alloys still have significant drawbacks. A recognized Ni.sub.3 Al alloy for structural use at both ambient and high temperatures in hostile environments is one designated as IC-221M. This alloy has a composition of, by atomic percent, 15.9 aluminum; 8.0 chromium; 0.8 molybdenum; 1.0 zirconium; and 0.04 boron. This alloy has many attractive qualities, including good strength, oxidation resistance, and wear resistance at elevated temperatures.
This alloy has become a standard for advanced material for use in die and tool applications, that is, hot forging. Despite its current use commercially, however, this alloy exhibits significant reductions in strength at temperatures in excess of about 800.degree. C. This limits its usefulness in both structural applications and tool and die applications requiring or existing in higher temperatures. Additionally, and especially in stamping and tooling operations, increased hardness would improve not only the use in existing applications but enable use in other applications as well.